1. Field of the Invention
Embodiments of the present invention generally relate to thermal processing of semiconductor substrates. In particular, the invention relates to laser thermal processing of semiconductor substrates.
2. Description of the Related Art
Thermal processing is required in the fabrication of silicon and other semiconductor integrated circuits formed in silicon wafers or other substrates such as glass panels for displays. The required temperatures may range from relatively low temperatures of less than 250° C. to greater than 1000° C., 1200° C., or even 1400° C. and may be used for a variety of processes such as dopant implant annealing, crystallization, oxidation, nitridation, silicidation, and chemical vapor deposition as well as others.
For the very shallow circuit features required for advanced integrated circuits, it is desired to reduce the total thermal budget in achieving the required thermal processing. The thermal budget may be considered as the total time at high temperatures necessary to achieve the desired processing temperature. The time that the wafer needs to stay at the highest temperature can be very short. For example, Rapid thermal processing (RTP) uses radiant lamps which can be very quickly turned on and off to heat only the wafer and not the rest of the chamber. Pulsed laser annealing using very short (about 20 ns) laser pulses is effective at heating only the surface layer and not the underlying wafer, thus allowing very short ramp up and ramp down rates.
A more recently developed approach in various forms, sometimes called thermal flux laser annealing or dynamic surface annealing (DSA), uses a tapered light pipe and anamorphic imaging optics to generate very intense beams of light that strike the wafer as a thin long line of radiation. The line is then scanned over the surface of the wafer in a direction perpendicular to the long dimension of the line beam. However, it has been reported that the light pipe used to homogenize and scale the image along the slow axis (i.e., the line length direction) is fragile, difficult to manufacture, and subject to misalignment to the other optics in the system.
Therefore, there is a need for a more efficient and economical optical system for projecting a laser line image that is less sensitive to alignment errors and less fragile.